Seventeen Republican candidates. Two debates. How does a contender in Thursday’s first matches of the 2016 presidential campaign measure success?

That’s an important, but subjective question, depending on the candidate, says GOP consultant Matt Mackowiak. “Success for Donald Trump is different than success for Marco Rubio.”

That said, he and others point to several “musts” for all the candidates, whether they are in the top 10 who get to debate in prime time at 9 p.m. on Fox News, or whether they're among the seven in the back of the bus who are relegated to the less desirable 5 p.m. slot.

Hands-down, the No. 1 goal of any candidate in a forum like this? Don’t mess up in a Twitterly bad way. Don’t go “oops,” like then-Gov. Rick Perry of Texas did in 2012 when he couldn’t remember all three government agencies he would shutter if he were president.

The aim in this crowded field is to break from the pack, and if you stumble on a gaffe, you’ve lost just steps from the starting block.

Which brings the candidates to the most important item on their to-do list: Find that breakout moment – in a rallying way. If you’re on the five o’clock “B” team, you want to be on the “A” team for that next debate. And if you’re with the A-listers, you want to get past The Donald, who’s leading the pack.

Newt Gingrich created his moment when he turned the tables on CNN’s John King in the 2012 Republican primary debate in South Carolina.

Mr. King had asked the presidential candidate about rumors of his open marriage, and Mr. Gingrich angrily replied that the question was “as close to despicable as anything I can imagine.” To which the audience loudly applauded.

“That was a very strong moment that to some extent propelled him to win South Carolina,” says Mr. Mackowiak. Of course, one breakout moment is not enough. A candidate also has to develop momentum to go the distance. Gingrich did not do that.

Donald Trump, for instance, will be successful if he can appear presidential, says Mackowiak. Jeb Bush will do well if he can show he’s got fire in his belly – and convince skeptics of his backing for immigration reform and Common Core education standards. Scott Walker can count it a plus if he shows mastery of a broad set of issues, including foreign policy.

It’s a question whether these debates will really matter all that much. Amy Black, a political scientist at Wheaton College in Illinois, says that debates have “little to no long-term effect” – at least that’s what studies show about one-on-one presidential debates.

“In the media, we talk about debates as if they are these major events and every person in America is stopping their life to sit in front of the TV,” says Professor Black. Millions are expected to tune in to this first debate, but people are also on vacation. They’re doing other stuff. What they mostly look at is media coverage afterward – including the lowlights and highlights (remember points 1 and 2 above).

Still, Black points out that this debate will impact all-important fundraising. If a candidate appears to do poorly, dollars will dry up. The reverse is true for candidates who come out shining.

And while it’s true that this debate may not mean much in the long term, it has meaning for this stage of the game. Says Mackowiak: “It matters in terms of who is still viable. Who has a clear path to the nomination. It has a clarifying effect.”